1. Field of the Invention
The present invention relates to a digital image data storage apparatus, a digital image data transfer apparatus and a digital image data broadcast apparatus including image an data encoder and decoder.
2. Description of the Prior Art
A Conventional encoding process for interlace-scanned image data for storing and transferring is shown in FIGS. 20A and 20B. FIG. 20A is a block diagram of a process of image data compression, and FIG. 20A is a block diagram of a process of image data decompression. For compressing interlace-scanned image data, original image data should be generated line by line from each field, the composite image data is divided into N*N pixel blocks, then those blocks are encoded by the method of the Discrete Cosine Transformation (DCT), which is a two-dimensional orthogonal transformation. Usually signal energy of the image data is biased at low frequency, and it is expected that high frequency components are transformed to "0" by applying the DCT and proper quantization. Therefore, highly efficient image data compression can be achieved by nullifying those components.
However, if objects in the image data exhibit rapid motion, the first field and second field of the composite image data are less related. Therefore, non-"0" components can appear more in the high frequency range even after applying the DCT and proper quantization. The total amount of available bits allotted as bits for coding the compressed data is fixed in the encoding process, and compressing efficiency may be decreased by allotting the available bits to those non-"0" components in the high frequency range.
Therefore, in case when objects in the image data exhibit rapid motion, better compression efficiency can be achieved by applying the DCT to each field independently. For example, digital video recorders usually have a function for selecting the frame DCT or the field DCT in advance by detecting that the block data is "static block data" with less motion or "motion block data" with more motion.
However, there is a problem in the above-mentioned conventional method in case the image data with more motion is captured and displayed as static image data on a monitor.
Big gaps at the edge of an image in the horizontal direction between each field will appear in the motion block data of interlace-scanned image data as shown in FIG. 22. Therefore, if it is displayed as a static image on the monitor without any adjustment, the displayed image will be deformed at the edge. In a worst case, the original image cannot be recognized because of this deformation.
One method to settle this problem is shown in JP Laid Open Patent Application No.H5-30496. According to Application No.H5-30496, gaps are compensated for by means of sliding and matching both fields spatially in the horizontal direction for reducing the deformation.
However, spatial matching of both fields requires motion vectors between fields. Calculation of the motion vectors using hardware requires special additional circuits, which lead to cost increases. Similarly, calculation of the motion vectors using software leads to processing time increases.